Addressing the nation on Monday morning, Trump said the country is "joined together today in sadness, shock, and grief" in the wake of the deadliest mass shooting in U.S. history.

The president praised the "true professionalism" of Las Vegas police in responding to the attack, which left at least 50 people dead and 400 injured.

“To the families of the victims, we are praying for you and we are here for you, and we ask God to help you through this very dark period,” he said.

Trump emphasized the need for unity to provide support for the families of the victims. He said he will travel to Las Vegas Wednesday to meet with law enforcement, first responders, and victims.

“Though we feel such great anger at the senseless murder of our fellow citizens, it is our love that defines us today,” he said.

Trump has ordered flags to be flown at half-staff until sunset on October 6 out of respect for the victims.

Earlier Monday, the president had tweeted a statement of sympathy.

"My warmest condolences and sympathies to the victims and families of the terrible Las Vegas shooting," he tweeted. "God bless you!"

As of 9 a.m. ET Monday, police said at least 50 people were dead and 400 injured after a gunman opened fire on the crowd at a country music concert on the Las Vegas Strip. The shooter, identified as 64-year-old Stephen Paddock, was positioned at a window on the 32nd floor of the Mandalay Bay Casino. He died at the scene after a confrontation with police.

“The President has been briefed on the horrific tragedy in Las Vegas. We are monitoring the situation closely and offer our full support to state and local officials. All of those affected are in our thoughts and prayers,” Press Secretary Sarah Sanders said in a separate statement.

Vice President Mike Pence told victims of the "senseless violence" that "the hearts & prayers of the American people," and he thanked first responders for their bravery.

According to the Department of Homeland Security, there are no known credible threats against other public venues at this time.

"Our collective hearts are breaking for the victims and their families," tweeted Ivanka Trump.

First Lady Melania Trump also offered prayers for the victims.

House Minority Leader Nancy Pelosi, D-Calif., released a similar statement of support.

Rep. Dina Titus, D-Nev., who represents most of Las Vegas said her office is working closely with local and federal authorities to provide support.

"We are a resilient and benevolent town that will not be intimidated by acts of violence," Titus said.

Sen. Dean Heller, R-Nev., said he has been in touch with the state's governor and attorney general and is monitoring the situation.

Sen. Catherine Cortez Masto shared information for those interested in donating blood for victims of the "senseless tragedy."

The chairman of the Senate Homeland Security Committee, Sen. Ron Johnson, R-Wis., said his committee will provide support the federal investigation of the shooting.

“My heart goes out to the victims, their families and the brave men and women who risked their lives to save others," he said in a statement. "My committee will work to support the federal role in investigating this attack and protecting against further threats. As Americans we must unite to defeat threats to our nation's security – both foreign and domestic.”